As an example of the conventional information recording and reproducing devices, the following description will discuss a magneto-optical memory device for recording, erasing, and reproducing on and from a magneto-optical disk.
As shown in FIG. 61(a), the magneto-optical disk is fabricated by forming a recording magnetic film 2805 on a disk substrate 2804 thereof. The recording magnetic film 2805 is formed so as to have an axis of easy magnetization whose direction is perpendicular to the film surface, and is initialized so that the directions of magnetization indicated by arrows in the recording magnetic film 2805 may direct a predetermined direction (for example, direction of magnetization A in FIG. 61(a)).
In a recording process, a laser beam 2803 projected from a semiconductor laser 2801 is applied to the recording magnetic film 2805 with a diameter of substantially 1 .mu.m focused by an objective lens 2802. In this example, luminous intensity of the laser beam 2803 is controlled according to a recording signal 2807 (shown by (b) in the figure) corresponding to information to be recorded. A local portion on the disk on which a laser beam with strong luminous intensity has been applied has a temperature rise to exceed the Curie temperature, and coercive force in the portion is lowered extremely. As a result, a direction of magnetization in the portion where the coercive force has been lowered is reversed so as to have the same direction as that of an externally applied magnetic field 2806 (direction of magnetization B in FIG. 61(a)) which is applied simultaneously as the projection of the laser beam 2803. In this manner, the information corresponding to the recording signal 2807 is recorded on the recording magnetic film 2805. The portions on which the magnetization with the direction B is recorded in the above-mentioned manner are hereinafter called "marks" 2809 while the other portions having the magnetization with the direction A is recorded are called "non-marks" 2810. An erasing process of the information recorded on the recording magnetic film 2805 is performed by a similar manner in the recording process, wherein the direction of the externally applied magnetic field 2806 is reversed to that in the recording process by returning the direction of the magnetization to the original direction when initialized (direction of magnetization A in FIG. 61). As a result, the portions on which the erasing process is performed become non-marks 2810.
In the present example, the light modulation method is adopted, i.e. recording is executed by modulating the intensity of the laser beam 2803 in accordance with the recording signal 2807, and applying an external constant magnetic field. However, the magnetic modulation method may as well be adopted and recording can be executed by making the luminous intensity of the laser beam 2803 constant and modulating the direction of the external magnetic field in accordance with the recording signal 2807.
The disk substrate 2804 mentioned earlier is made of glass, plastic or other material. Protrusions and recessions 2808 are preliminarily etched on the disk substrate 2804, as shown in FIG. 61(a). The protrusions and recessions 2808 represent address data indicating the addresses of tracks and sectors.
The above address data are preliminarily etched onto the disk substrate 2804 according to a predetermined format. Hence, they cannot be recorded or erased thereafter. Hereinafter, areas where a plurality of protrusions and recessions 2808 are formed in a group will be referred to as pre-formatted sections 3003. On the other hand, information is recorded and erased in areas other than the pre-formatted sections 3003. These areas will be referred to hereinafter as MO (magneto-optical) sections 3002. As shown ill FIG. 63, pre-formatted sections 3003 and MO sections 3002 are usually disposed alternately on a track 3005 formed in a spiral shape or in the shape of concentric circles. A sector 3004 is constituted by a pair composed of a pre-formatted section 3003 and MO section 3002. A magneto-optical disk 3001 comprises a plurality of sectors 3004 formed on the track 3005. Each sector 3004 is provided with address data, and information is recorded, reproduced and erased by each sector 3004.
As illustrated in FIG. 64, the pre-formatted sections 3003 of the tracks 3005 are arranged such that either the recessions or the protrusions that compose the protrusions and recessions 2808 shown in FIG. 61(a) form marks 2811, and such that the other components of the protrusions and recessions 2808 form non-marks 2812. Marks 2809 and non-marks 2810 are recorded in the MO section 3002 in response to MO signals as described earlier.
When reproduction is performed on the magneto-optical disk 3001, a laser beam 2803 projected from a semiconductor laser 2801 is irradiated upon the recording magnetic film 2805 with a diameter of 1 .mu.m, as illustrated in FIG. 62(a). Here, the luminous intensity of the laser beam 2803 is weaker when information is reproduced than when information is recorded or erased. The laser beam 2803 is a linearly polarized light and its plane of polarization is rotated as the laser beam 2803 passes through or is reflected by the recording magnetic film 2805 due to the Faraday effect or the Kerr effect. The plane of polarization of the laser beam 2803 is rotated in mutually opposite directions depending on whether the laser beam 2803 is projected on a mark 2809 or a non-mark 2810. Reproduction is performed by detecting the difference in polarization direction. Consequently, two types of reproduced signals S1 and S2, shown by (b) and (c) in FIG. 62, are generated.
Referring to FIG. 65, the reproduction optical system employed for producing the reproduced signals S1 and S2 will be described briefly below.
As illustrated in FIG. 65, a reflected light 3201 from the magneto-optical disk 3001 is directed toward a PBS (analyzer) 3202 where it is split into two detected lights 3210 and 3211 according to its polarization direction. The two detected lights 3210 and 3211 are respectively directed toward photodetectors 3203 and 3204 where they are converted into electric signals that vary according to the respective luminous intensities of the detected lights 3210 and 3211, and released as reproduced signals S1 and S2. As it will be described in detail later, the signals from the pre-formatted section 3003 and the MO section 3002 are obtained separately by determining the sum and the difference of the reproduced signals S1 and S2. In addition, the marks 2809 and the non-marks 2810 may be reproduced separately through the signals of the MO section 3002 thereby enabling the information recorded on the recording magnetic film 2805 to be reproduced.
As illustrated in FIG. 66(a), assume that .alpha. represents the vector of a reflected light from a non-mark 2810 (direction of magnetization A) of the MO section 3002, and .beta. represents the vector of a reflected light from a mark 2809 (direction of magnetization B) of the MO section 3002. The reflected light vectors .alpha. and .beta. are rotated in opposite directions by an angle corresponding to the rotation angle of their respective plane of polarization. The X direction components and Y direction components of the reflected light vectors .alpha. and .beta. are detected in the analyzer (PBS) 3202 that transmits light having a X or Y polarization direction. These two polarization directions X and Y form a right angle. The reflected light vector .alpha. is projected in the polarization direction X and the polarization direction Y thereby producing detected light vectors .alpha..sub.X and .alpha..sub.Y. Similarly, the reflected light vector .beta. is projected in the polarization detection X and the polarization direction Y thereby producing detected light vectors .beta..sub.X and .beta..sub.Y. The magnitudes of detected light vectors .alpha..sub.X and .beta..sub.Y correspond to reproduced signals S1 and the magnitudes of the detected light vectors .alpha..sub.X and .beta..sub.Y corresponds to reproduced signals S2. Further, the detected light vectors .alpha..sub.X .multidot..beta..sub.X and .beta..sub.Y .multidot..alpha..sub.Y respectively correspond to the two kinds of detected light 3210, 3211 shown in FIG. 65.
As shown in FIG. 66(a), the high level of the reproduced signal S1 corresponds to a non-mark 2810 and the low level of the reproduced signal corresponds to a mark 2809. As to the reproduced signal S2, its low level corresponds to a non-mark 2810 and its high level to a mark 2809. The polarity of the reproduced signal S1 and the polarity of the reproduced signal S2 are opposite to each other. The reproduced signals S1 and S2 are then fed into a differential amplifier where the reproduced signals S1 and S2 are differentially amplified and their S/N ratios are improved, and information is reproduced.
Reproduced signals S1 and S2 obtained from the pre-formatted sections 3003 will be described below with reference to FIG. 66(b). As there is no recording nor erasing operation taking place in the pre-formatted sections 3003, the direction of magnetization therein coincides with the direction A only. When the laser beam is irradiated on a pre-formatted section 3003, the shape of the marks 2811 and non-marks 2812, i.e. the protrusions and recessions 2808 causes the laser beam to be diffracted. As a result, a long reflected light vector .alpha. (corresponding to the reproduction of a non-mark 2812) or a short reflected light vector .gamma. (corresponding to the reproduction of a mark 2811) is produced according to the protrusions and recessions 2808, as illustrated in FIG. 66(b).
An analyzer vector .alpha..sub.X .multidot..gamma..sub.X and an analyzer vector .gamma..sub.Y .multidot..alpha..sub.Y are produced by, projecting these reflected light vectors in the polarization directions X and Y of the analyzer (PBS) 3202. The magnitudes of the analyzer vector .alpha..sub.X .multidot..gamma..sub.X and of the analyzer vector .gamma..sub.Y .multidot..alpha..sub.Y correspond to the reproduced signals S1, S2. The high level of the reproduced signals S1 and the high level of the reproduced signals S2 both correspond to non-marks 2812 of the protrusions and recessions 2808, the low level of the reproduced signals S1 and the low level of reproduced signals S2 correspond to marks 2811. Accordingly, different from the mark 2809 and the non-mark 2810 for the magneto-optical recording in the case shown in FIG. 66(a), the reproduced signals S1, S2 have the same polarity. More specifically, as illustrated in FIGS. 66(b) and (c), the reproduced signals S1 and S2 have the same polarity for the pre-formatted section 3003 while they have mutually inverted polarities for the MO section 3002.
Accordingly, by adding the reproduced signals S1 and S2, only the signal corresponding to the pre-formatted section 3003 may be obtained, and by subtracting the reproduced signal S1 from S2, only the signal corresponding to the MO section 3002 may be obtained. Thus, the S/N ratio may be improved.
When the differential signal or the sum signal from the above-mentioned two types of reproduced signals S1 and S2 are amplified by an AGC amplifier with their amplification degree being adjusted therein, the amplitudes after the amplification become substantially constant.
As shown by a reference numeral 10 in FIG. 67, the AGC amplifier, for example, comprises a voltage control amplifier (hereinafter called VCA) 11 whose amplification degree varies according to a control voltage (hereinafter called AGC voltage), a signal level detection circuit 12 for generating a voltage in accordance with the level of a signal released from VCA 11 and an operational amplifier 13 for releasing the AGC voltage which is directly proportional to the difference between the level of the output signal from the VCA 11 detected by the signal level detection circuit 12 and the level of a reference signal preliminarily set. With the arrangement, a feedback control is performed so as to keep the level of the reproduced signal substantially constant by entering the AGC voltage released from the operational amplifier 13 to the VCA 11.
However, such a conventional magneto-optical recording and reproducing device has the drawback that reproducing operation is not always performed appropriately due to the fact that the amplification degree of the AGC amplifier becomes extremely small immediately after a recording or erasing operation. The following description will discuss the drawback referring to FIG. 68.
Here, assume that at a sector 3004 shown by A in FIG. 68(a), a reproducing operation of a pre-formatted section 3003 and a recording operation of an MO section 3002 are performed, and that at a sector 3004 shown by B, a reproducing operation is performed, and that at a sector 3004 shown by C, a reproducing operation of a pre-formatted section 3003 and an erasing operation of information recorded on an MO section 3002 are performed.
In that case, at the sector 3004 shown by A, since light amount of the semiconductor laser 2801 (see FIG. 61) increase to a quantity corresponding to the recording light amount as aforementioned, the amplitude of the reproduced signals S1, S2 at the MO section 3002 become extremely great, as shown in FIG. 68(b) (here, the reproduced signals S1, S2 are signals obtained from reflected lights derived from the laser beam 2803 for recording which is reflected from the surface of the magneto-optical disk 3001). As a result, since the AGC voltage starts increasing as shown in FIG. 68(d) while the amplification degree of the VCA 11 starts decreasing, the level of the output signal of the VCA 11 gradually lowers as shown in FIG. 68(c).
However, after the above process, when the relevant process comes to the pre-formatted section 3003 of the sector 3004 shown by B or C to perform a reproducing operation thereof, since the amplification degree of the VCA 11 has still remained small, the amplitude of the reproduced signal of the pre-formatted section 3003 becomes extremely small after having been amplified by the VCA 11 as shown by B.sub.1 and C.sub.1, thereby causing a possibility of reproduction error.
Similarly, when an erasing operation is performed on the sector 3004 shown by C, since light amount of the semiconductor 2801 becomes greater in comparison with that in reproducing operation, the amplitude of the reproduced signals S1 (S2) becomes great as shown in FIG. 68(b) (here, the reproduced signals S1 (S2) are signals obtained from reflected lights derived from the laser beam 2803 for erasing which is reflected from the surface of the magneto-optical disk 3001). As a result, since the amplification degree of the VCA 11 becomes small, the level of the output signal of the VCA 11 also becomes small in reproducing the pre-formatted section 3002 of the sector 3004 shown by D (see D.sub.1 section), and consequently an accurate reproducing operation may not be obtained.
Furthermore, in access operation in the magneto-optical recording and reproducing device, as shown in FIG. 69, a laser beam 2803 moves on the disk substrate 2804 in the radius direction shown by arrow P or Q while transversing the tracks 3005 disposed in the form of guiding grooves on the disk substrate 2804 or the protrusions and recessions 2808 at pre-formatted sections. In that case, since an amount of the reflected light is different between in portions including the tracks 3005 or protrusions and recessions 2808 and in other portions not including those, a reproduced signal S1 or S2 shown by A in FIG. 70 may be obtained. On the other hand, a reproduced signal S1 or S2 in reproducing information is shown by B in FIG. 70.
Thus, in access operation, the amplitude of the reproduced signal S1 or S2 sometimes becomes greater or smaller than that in reproducing operation for information. The amplitude varies depending on the depth and width of the guiding grooves. According to the variation, the amplification degree of the AGC amplifier sometimes becomes great or small. As a result, there is also a possibility of reproduction error also in reproducing operation for information immediately after an access operation. Additionally, when an access speed is comparatively slow, by providing a high-pass filter in the processing system for the reproduced signals S1 or S2, oscillatory components caused by the tracks 3005 or protrusions and recessions 2808 may be eliminated, thereby reducing an occurrence of reproduction error immediately after an access operation. However, since it is normally desirable to make the access speed as fast as possible, it is difficult to eliminate reproduction errors due to the decrease or increase of the amplification degree of the AGC amplifier immediately after an access operation.
The reproduction errors immediately after the recording, erasing, or access operation may be reduced by making the AGC amplifier respond quickly to the variation of the amplitude of the input signal.
However, in the case where the response speed of the AGC amplifier is set extremely fast, for example, when a defect pulse occurs because of a damaged portion, dust or the like on the magneto-optical disk 3001, a reproduction error tends to occur since the amplification degree of the AGC amplifier immediately becomes small in response to the defect pulse. In addition, the response speed of the AGC amplifier is originally set low in order to reduce the reproduction errors due to the defect pulses.
As aforementioned, in either case where the response speed of the AGC amplifier is set low or is set high, the reproduction errors tend to occur due to the respective different reasons. Basically, it is preferable to set the response speed of the AGC amplifier low so as to minimize the influence of the defect pulses; however, in the arrangement, the reproduction errors due to the response delay of the AGC amplifier are not avoidable.
In the meantime, MO sections 3002 of all the sectors 3004 on the magneto-optical disk 3001 do not necessarily have information made of MO signals recorded therein, and normally there exist some MO sections 3004 which have no information recorded therein.
In that case, when an instantaneous reproducing position is located in a sector 3004 with no information recorded therein, the AGC amplifier 10 responds to a low-level reproduced signal without a group of pulses made of MO signals, thereby making the amplification degree of the VCA 11 become extremely great. After this process, when the instantaneous reproducing position comes to reach a sector 3004 with information recorded therein and a reproduced signal with a group of information pulses made of MO signals is about to be entered to the VCA 11, the amplification degree may not immediately follow the level of the reproduced signal.
More specifically, immediately after the reproduced signal with the pulse signals (see FIG. 71(a)) have been entered to the VCA 11, the level of the output signal of the VCA 11 increases sharply as shown in FIG. 71(b) as an example. As a result, a problem is presented in that a normal reproduced signal may not be obtained during the time until the amplification degree achieves an appropriate degree by following the level of the reproduced signal.
On the other hand, in the case where the magneto-optical disk 3001 has a scratch or dust on the surface thereof, the input signal of the VCA 11 sometimes contains a defect pulse 41 in an un-recorded area as shown in FIG. 72(a). When the AGC amplifier responds to the defect pulse 41, the amplification degree of the VCA 11 becomes extremely small. It normally takes more time to recover an appropriate amplification degree from the amplification degree which has become extremely small than to recover it from the amplification degree which has become extremely great.
Thus, as shown by II in FIG. 72(b), after the occurrence of the defect pulse 41, the level of the output signal of the VCA 11 decreases sharply for a long period. Therefore also in this case, a normal reproduced signal may not be obtained.
Furthermore, in order to restrain the variation of the amplification degree of the AGC amplifier 10 due to the existence of MO sections 3002 with no information recorded therein on the magneto-optical disk 3001, it is proposed to install a pulse group detection circuit for detecting whether there is a group of pulses in the MO sections 3002 or not. With the arrangement, in an un-recorded area having MO sections without a group of information pulses, an AGC voltage immediately before the reproduction in the un-recorded area is held so as to keep the amplification degree of the VCA 11 constant, and when the next recorded area having a group of information pulses is reproduced, the holding operation of the AGC voltage is released. In this manner, the irregular variation of the amplification degree of the AGC amplifier 10 due to the existence of un-recorded areas with no information recorded therein is substantially restrained.
In that case, however, the holding operation of the AGC voltage is apt to be released by mistaking the defect pulse 41 for a group of information pulses. Especially in a magneto-optical disk recording and reproducing device whose AGC amplifier 10 is preliminarily set so as to have a maximum amplification degree upon its resetting operation, the possibility that the AGC amplifier 10 might respond to the defect pulse 41 becomes higher immediately after the resetting operation of the AGC amplifier 10 or immediately after the start of the magneto-optical disk recording and reproducing device.
In that case, the amplification degree of the AGC amplifier 10 varies according to an amplitude of the defect pulse 41, and the amplification degree is fixed at the end of the defect pulse 41. Accordingly, when the reproduction of information in the next recorded area is performed, a problem is apt to occur in reproducing information derived from MO signals. Especially, in the case of great amplitude of the defect pulse 41, since the amplification degree of the AGC amplifier 10 is fixed to be extremely small at the end of the defect pulse 41, the information recorded therein might not be reproduced due to the extremely small amplification degree during the next recorded area. Thereafter, since the amplification degree is kept at the extremely small degree which was fixed at the end of the defect pulse 41, the reproducing operation of information may not be executed until the AGC amplifier 10 or the magneto-optical disk recording and reproducing device is reset in the next occasion.
As aforementioned, although reliability of the AGC amplifier 10 may be improved to a certain extent by the use of the pulse group detection circuit, the arrangement may not provide a decisive solution to the defect pulse 41 or other problems.
Additionally, in general it has been known that those defect pulses occur more frequently in an optical memory such as magneto-optical disk than in a magnetic disk. Accordingly, it is necessary to prevent the occurrence of errors in reproducing information, especially due to defect pulses.
The following description will discuss a reproduction circuit of the magneto-optical disk recording and reproducing device referring to FIG. 73 and FIG. 74.
As shown in FIG. 73, the reproduced signals S1 and S2 are entered to a reproduction circuit 3501, and a binary coded output signal 3510 therefrom is entered to an address generation circuit 3502 and a timing generation circuit 3503. In the address generation circuit 3502, address information in the pre-formatted section 3003 of each sector shown in FIG. 63 is read from the output signal 3510, and an address signal 3511 is released therefrom. On the other hand, in the timing generation circuit 3503, a sector mark for synchronizing sectors, which is also located in the pre-formatted section, is detected, and a reference timing signal 3512 for recording, reproducing, or erasing is released therefrom.
Referring to FIG. 74, the following description will discuss a pre-formatted waveform processing circuit which is disposed in the conventional reproduction circuit 3501 of FIG. 73. The reproduced signals S1 and S2 are entered to a summing amplifier 3602. In the summing amplifier 3602, the information derived from the MO (data) section where the polarities of the reproduced signals S1 and S2 are opposite to each other is removed, and only the information derived from the pre-formatted section where those signals have the same polarity is separated to form an output signal 3605 therefrom. The output signal 3605 of the pre-formatted section thus obtained is entered to an AGC amplifier 3603. According to an AGC voltage released from an AGC voltage generation section (not shown) in an AGC circuit, the AGC amplifier 3603 controls its amplification degree so as to have a predetermined signal level as a reproduced signal. An output signal 3606 of the AGC amplifier 3603 is entered to a binary code circuit 3604 to be converted into a binary coded signal, and released as the binary coded signal 3510. Then, from the binary coded signal 3510, as shown in FIG. 73, an address signal 3511 and a reference timing signal 3512 for recording, reproducing, or erasing are obtained in an address generation circuit 3502 and a timing generation circuit 3503 respectively. The magneto-optical disk recording and reproducing device performs a recording, reproducing or erasing operation of information on or from a sector having a desired address according to the address signal 3511 and the reference timing signal 3512.
However, the above-mentioned conventional device using the AGC amplifier 3603 shown in FIG. 74 sometimes fails in reproducing address information in a pre-formatted section 3003. Therefore, each recording, reproducing or erasing operation can not be performed because of the failure in finding a sector 3004, where information is to be recorded, reproduced or erased. The following description will discuss the above drawback referring to FIG. 75 and FIG. 76. FIG. 75 shows shapes of the marks 2808 in the pre-formatted section 3003 on the magneto-optical disk and amplitudes of reproduced signals obtained according to the respective shapes. The pre-formatted section 3003 includes a sector mark section 1701 having sector synchronous information recorded therein and an ID section 1702 having address information recorded therein, and each piece of information is preliminarily recorded in the form of a protrusion and a recession physically etched. As shown in FIG. 75(a), the sector mark section 1701 consists of a series of marks 2808 with a shape of elongated circle which has an apparently different arrangement from that of other information located in ID section and data section. On the other hand, as shown in FIG. 75(b), the ID section 1702 consists of a series of marks 2808 with a shape of circle which may be obtained, for example, by 2-7 modulation method (which will be described later). As shown in FIG. 75( c) and (d), depending on the different shapes of the marks, an amplitude of the reproduced signal derived from the sector mark section 1701 having the marks 2808 with a shape of elongated circle becomes greater than that of the reproduced signal derived from the ID section having the marks 2808 with a shape of circle, and for example, becomes twice as great as that. This is because, as the total area of the mark 2808 within a laser spot 2701 becomes closer to the total area of non-mark within the laser spot 2701, interference between reflected lights from the laser beam becomes greater, and therefore makes the amplitude of the reproduced signal also become greater. Further, the amplitude varies depending on the width and depth of the mark. For example, waveforms shown in FIG. 75 are respectively obtained by reproducing the sector mark section 1701 and the ID section 1702 both of whose marks 2808 have the depth of substantial 100 nm and the width of substantial 0.4 .mu.m by the use of a laser spot having the diameter of 1.3 .mu.m (which is a Gaussian beam). Therefore, if the depth or width of the mark 2808 is changed or if the diameter of the laser spot 2701 is changed, the waveforms shown in FIG. 75 will change.
Referring to FIG. 76, the following description will discuss the fact that, when the reproduced signals of the pre-formatted section 3003 including the signal of the sector mark section 1701 having a greater amplitude and the signal of the ID section 1702 having a smaller amplitude are entered to the pre-formatted waveform processing circuit shown in FIG. 74, the conventional AGC amplifier 3603 does not respond to the amplitude of the signal derived from the ID section 1702.
As shown in FIG. 76(a), the pre-formatted section 3003 includes the sector mark section 1701 and the ID section 1702. For the aforementioned reason, an amplitude 3801 of the reproduced signal of the sector mark section 1701 is greater than an amplitude 3802 of the reproduced signal of the ID section 1702 as shown in FIG. 76(b). Those reproduced signals are entered to the AGC amplifier 3603 after having been removed their dc components by ac coupling. An output signal after the ac coupling has a waveform shown in FIG. 76(c) due to a transient response characteristic of the ac coupling. In the AGC amplifier 3603, an AGC voltage is generated from an AGC voltage generation section not shown in the figure, responding to the amplitudes of the input signals, and the output signal is automatically controlled in its amplification degree by the AGC voltage so as to have a predetermined amplitude.
However, in the case of the reproduced signals of the pre-formatted section 3003, where the signal of the ID section 1702 with a small amplitude follows immediately after the AGC amplifier having responded to the signal of the sector mark section 1701 with a great amplitude, the amplification degree still remains responding to the amplitude of the signal from the sector mark section 1701 because the AGC voltage does not change instantly. Accordingly, the amplification degree remains low to the signal of the ID section 1702. Originally, it is necessary to make the AGC amplifier respond to the amplitude of the signal from the ID section 1702 when the signal of the ID section 1702 is reproduced.
The conventional AGC amplifier 3603, however, does not respond to the amplitude of the signal of the ID section 1702. Moreover, a ratio of the amplitude of the signal of the sector mark section 1701 to the amplitude of the signal of the ID mark section 1702 is not necessarily constant. For that reason, as shown in FIG. 76(d), the amplitude of the signal from the ID section 1702 in the output signal of the AGC amplifier 3603 is small and is not constant, and consequently address information located therein may not be reproduced. Thus, a problem is presented in that information in a desired sector 3004 may not be retrieved.
In the meantime, in the aforementioned magneto-optical recording system, the sizes of the marks are changed according to each of recording conditions such as the recording light amount, recording pulse length, and external magnetic field. The following description will discuss the relationship between the recording conditions and the sizes of the marks to be recorded under the conditions, referring to FIG. 77.
As shown in FIG. 77(a) and (b), the size of the mark to be recorded becomes greater as the amplitude of the recording pulses, that is, the recording light amount increases, (supposing that each of the recording pulses has a given length). On the other hand, as shown in FIG. 77(c) and (d), when the length of the recording pulse (pulse width) is increased with its amplitude kept at a given value, the size of the mark to be recorded also becomes greater in directly proportional to the length of the pulses.
If the recording conditions are not properly controlled, an error sometimes occurs in reproducing data in a reproducing operation. The following description will discuss the occurrence of the error in reproducing data referring to FIGS. 78 to 85.
FIG. 78 shows a main arrangement of an MO waveform processing section 2502, and the reproduced signal obtained by differentially amplifying the signals S1 and S2 is entered to an equalizer 50 in the MO waveform processing section 2502. An output of the equalizer 50 is released from a zero-crossing detection circuit 52 as a differentiating zero-crossing detection signal through a differentiating circuit 51. Further, the output of the equalizer 50 is also entered to a level detection circuit 53 from which it is released as a gate signal. Referring to FIG. 79, the following description will discuss a reproducing operation in the MO waveform processing circuit 2502 shown in FIG. 78.
According to modulated data shown in FIG. 79(a) (for example, data modulated by 2-7 modulation method), marks are recorded (in magneto-optical recording) as shown in FIG. 79(b). In a reproducing operation, when a laser spot 2701 having a diameter indicated by an alternate long and short dash line in FIG. 79(b) is applied onto the MO section where those marks have been thus recorded, a reproduced signal (a differential signal or a sum signal of reproduced signals S1 and S2, in the case of a pre-formatted section) is reproduced, which has a maximum amplitude, for example, at the center of the recorded mark as shown in FIG. 79(c). As shown in FIG. 79(c), in the places where a distance between the marks is short, a peak to peak value of the reproduced signal is small, and the frequency components thereof are high. The reproduced signal has its high frequency component emphasized in the equalizer 50, and has an equalizer output signal waveform as shown in FIG. 79(e). The equalizer output signal is differentiated in the differentiating circuit 51 from which it is released as a differentiating signal as shown in FIG. 79(d). Further, from the differentiating signal, a differentiating zero-crossing detection signal is formed and generated in a zero-crossing detection circuit 52 as shown in FIG. 79(f). Additionally, zero-crossing noise in the differentiating zero-crossing detection signal is produced at a portion in a vicinity of zero-level where a rate of change is small in the differentiating signal as shown in FIG. 79(d). Moreover, as shown in FIG. 79(g), a gate signal derived from the equalizer output signal is generated from a level detection circuit 53. When a falling edge of the differentiating zero-crossing detection signal is detected during the period the gate signal is in the high level, a data signal rises from the low level to the high level, and falls to the low level upon the end of the gate signal, as shown in FIG. 79(h). With the above process, reproduction data may be obtained by reproducing the modulated data (see FIG. 79(i)).
On the other hand, as shown by broken lines in FIG. 79(b), when the marks are recorded under different recording conditions, an S/N ratio deteriorates and Jitters in the differentiating zero-crossing detection signal increase according to the S/N ratio as shown by broken lines in FIG. 79(f). Further, although not shown in FIG. 79(e), the S/N ratio of the equalizer output signal also deteriorates, and therefore jitters in the gate signal also increase according to the ratio. Accordingly, jitters occur in the reproduction data, and its S/N ratio deteriorates. In addition, the above-mentioned data signal is produced, for example, by a gate circuit 54 as shown in FIG. 80.
More specifically, the differentiating zero-crossing detection signal is transmitted to a clock input terminal CLK of a flip-flop 56 through an inverter 55 in the gate circuit 54. When the differentiating zero-crossing detection signal falls from the high level to the low level, the input of the clock input terminal CLK rises from the low level to the high level. As a result, an output terminal Q of the flip-flop 56 changes from the low level to the high level, and holds the condition until the gate signal falls to the low level. In other words, when the gate signal becomes the low level, it is transmitted to a clear terminal CL of the flip-flop 56, and therefore the output of the output terminal Q becomes the low level. Consequently, the data signal is produced, which is kept in the high level synchronizing to the falling edge of the differentiating zero-crossing detection signal, only during the high level of the gate signal.
As an example of the marks in different recording methods, there is shown a case wherein information is recorded by the use of edges of the marks. In this case, a circuit configuration shown in FIG. 82 is used as a MO waveform processing section.
The MO waveform processing section comprises a cosine equalization circuit 57, a comparison voltage generation circuit 58 and a comparator 59. The reproduced signal is entered to the cosine equalization circuit 57. The output of the cosine equalization circuit 57 is released as a binary coded signal through the comparison voltage generation circuit 58 and the comparator 59. The following description will discuss a reproducing operation of the MO waveform processing section shown in FIG. 82, referring to FIG. 83.
Based on modulated data shown in FIG. 83(a), marks are recorded (in magneto-optical recording) as shown in FIG. 83(b). In a reproducing operation, when a laser spot, which is not shown in the figure, is projected onto the MO section where those marks have been thus recorded, a reproduced signal is reproduced, which has a great amplitude, for example, at the center of the recorded mark as shown in FIG. 83(c). As shown in FIG. 83(c), in the places where a distance between the marks is short, a peak to peak value of the reproduced signal is small, and the frequency components thereof are high. The reproduced signal has its high frequency components emphasized in the cosine equalization circuit 57, and has an cosine equalization output signal waveform as shown in FIG. 83(d). The cosine equalization circuit 57 shapes the waveform of the reproduced signal, while maintaining its quality. According to the cosine equalization output signal, the comparator 59 generates a binary coded signal shown in FIG. 83(e) by using the output of the comparison voltage generation circuit 58 as a threshold value. In addition, the comparison voltage generation circuit 58 may be constituted of, for example, a low-pass filter or an envelope detection circuit, and the threshold value varies according to the input of the comparison voltage generation circuit 58. By using the binary coded signal thus obtained and a PLL clock shown in FIG. 83(f), reproduction data (see FIG. 83(i)) is generated, for example, in a conversion circuit 60 shown in FIG. 81. Referring to FIG. 81 and FIG. 83, the following description will discuss the generation of the reproduction data, more specifically.
As shown in FIG. 81, the binary coded signal is entered to a data input terminal D of a first flip-flop 61 in the conversion circuit 60. The PLL clock is transmitted to each input terminal CLK of the first flip-flop 61 and a second flip-flop 62. Synchronizing to the rising edge of the PLL clock, the output of an output terminal Q of the first flip-flop 61 varies according to the binary coded signal. For example, when a binary coded signal shown in FIG. 83(e) is entered to the data input terminal D and a PLL clock shown in FIG. 83(f) is entered to the clock input terminals CLK, the signal from the output terminal Q of the first flip-flop 61 shapes a waveform as shown in FIG. 83(g). Since the signal with the waveform is entered to the data input terminal D of the second flip-flop 62, the output of the output terminal Q of the second flip-flop 62 shapes a waveform as shown in FIG. 83(h). Therefore, there is a phase shift by one clock of the PLL clock between the waveforms from the respective output terminals Q of the first flip-flop 61 and the second flip-flop 62.
Then, the outputs of the first flip-flop 61 and the second flip-flop 62 are respectively transmitted to input terminals of exclusive OR gate 63 disposed in the next stage thereof, and exclusive OR operation is performed to form reproduction data shown in FIG. 83(i), which is released from the gates 63.
On the other hand, when marks are recorded under different recording conditions as shown by broken lines in the figure, the phase of the reproduced signal shifts due to the conditions as shown by broken lines in FIG. 83(c). Further, although not shown in FIG. 83(d), since the phase of the cosine equalization output signal also shifts, this makes the phase of the binary coded signal shift following the deviation. Thus, those shifts cause jitters, and errors may occur in the reproduction data, thereby deteriorating the S/N ratio.
As aforementioned, in order to obtain reproduction data without errors in reproducing operation, recording conditions should be always controlled most appropriately. For optimum control on the recording conditions, it has been conventionally known that by test writing information having a predetermined reference frequency on a magneto-optical recording medium while changing recording light amount or other factors, and by finding conditions for permitting the reproduced signal to have a maximum amplitude thereof in reproducing operation, recording operation thereafter is performed under the conditions thus found. For example, as shown in FIG. 84(a) and (b), an S/N ratio is the most appropriate under the recording conditions wherein the marks shown by solid lines in the reproduced signal were recorded, and in recording operation, in either case where recording light amount (or recording pulse length) is greater than that shown by the solid lines or is smaller than that, the amplitude (peak to peak value) of the reproduced signal becomes smaller, as shown by broken lines in FIG. 24(b). (see, for example, Japanese Unexamined Patent Publication No. 80138/1983 (Tokukaisho 58-80138)).
As a circuit for detecting the amplitude of the reproduced signal, there has been known and adopted, for example, an envelope detection circuit 24 as shown in FIG. 85. The circuit 24 mainly comprises a buffer circuit 25, a first peak hold circuit 26, a second peak hold circuit 27 and a differential amplification circuit 28.
The reproduced signal is entered through the buffer circuit 25 to the first and second peak hold circuits 26, 27 where both of the upper and lower peak values thereof are hold, and then an amplitude level signal corresponding to the peak to peak value of the reproduced signal is released from the differential amplification circuit 28. The optimum recording conditions may be determined depending on the size of the amplitude level signal. As another method, there is an example wherein the optimum recording conditions are also determined by detecting an envelope or primary and secondary higher harmonic waves in the reproduced signal. (see, for example, Japanese Unexamined Patent Publication No. 193544/1984 (Tokukaisho 59-193544) or No. 13334/1985 (Tokukaisho 60-13334)).
However, the above conventional arrangement has a drawback that it is necessary to install extra circuits for detecting a maximum amplitude, an envelope, or primary and secondary higher harmonic waves of the reproduced signal in reproducing process, and it makes the circuit configuration more complicated, thereby resulting in a high manufacturing cost of the entire system. Moreover, the arrangement having an AGC circuit has a drawback that a holding operation has to be performed in the AGC circuit, and therefore its control becomes complicated, thereby making it difficult to provide a small size and low cost system.
Referring to FIG. 86, the following description will discuss a buffer amplifier 3601 disposed at an input stage of the conventional reproduction circuit 3501 of FIG. 73.
The reproduced signal S1 is entered to a high-pass filter comprising a capacitor 3602 and a resistor 3603, located in the buffer amplifier 3601. As shown in FIG. 86, a voltage V.sub.O is applied to one terminal of the resistor 3603. A dc component in the reproduced signal S1 is removed by the high-pass filter, thereby making it easy to reproduce only an information signal contained in an ac component. An output signal 3610 from the high-pass filter is entered to an amplifier 3604 from which an output signal 3612 thereof is transmitted to the reproduction circuit 3501 located at a stage downstream.
The reproduced signal S2 is also entered to a high-pass filter comprising a capacitor 3605 and a resistor 3606, located in the buffer amplifier 3601, and a dc component of the reproduced signal S2 is thus removed, thereby making it easy to reproduce only an information signal contained in an ac component. An output signal of the high-pass filter is transmitted to the reproduction circuit 3501 located at a stage downstream as an output signal 3613 through an amplifier 3607.
However, the above conventional device by the use of the buffer amplifier 3601 of FIG. 86 sometimes fails to read information in the pre-formatted section immediately after each of recording and erasing operations for information. The drawback becomes critical especially in performing high-speed transfers or high density recording for information. The following description will discuss the drawback referring to FIG. 87 and FIG. 88.
FIG. 87 and FIG. 88 show an example of a waveform of each section in the buffer amplifier 3601. As shown in FIG. 87(a), it is supposed that a recording operation for information is performed in a sector constituted of a pre-formatted section 3701 and an MO (data) section 3702, and a reproducing operation for information is performed in a sector constituted of a pre-formatted section 3703 and an MO (data) section 3704, and further, an erasing operation for information is performed in a sector constituted of a pre-formatted section 3705 and an MO (data) section 3706. Each of these recording, reproducing and erasing operations for information has to be executed while reading synchronous timing information and address information recorded in the pre-formatted sections 3701, 3703 and 3705, detecting each predetermined synchronous timing, and successively identifying whether the address is a predetermined one or not.
In the meantime, the reproduced signal S1 and S2 form a signal whose amplitude becomes great in each of the recording and erasing operations, as shown in FIG. 87(b). This is because in each of the recording and erasing operations, a reflected light with great luminous intensity is entered to photo detectors 3203, 3204. According to the above process, as shown in FIG. 87(c), the output signals 3612, 3613 from the buffer amplifier 3601 form a waveform which is influenced by a transient response of the high-pass filter. In other words, the level of the signal fluctuates up and down immediately after each of the recording and erasing operations. A reproducing operation must be performed in each of pre-formatted sections 3703, 3707 immediately after each of the recording and erasing operations.
On the other hand, the reproduction circuit 3501 including the buffer amplifier 3601 has its range of reproduction limit between its upper and lower reproduction levels (hereinafter called reproduction limit range) as shown in FIG. 87(c), and it is impossible to read information from the signals S1 (S2) if the signal level is located out of the range.
The above reproduction limit range indicates a range wherein the circuits are normally operable electrically, and for example, it shows a range at which the signal level does not become saturated. Further, if there is installed an AGC amplifier at a step downstream, it shows a range wherein reproduction is operable and the AGC amplifier can respond within a given time.
In that case, as shown in FIG. 87(c), the output signal 3612 (3613) from the buffer amplifier 3601 in the pre-formatted sections 3703, 3707 is located far away from the range immediately after each of the recording and erasing operations. Since a dc component of the reproduced signal S1 (S2) especially in erasing operation is much greater than that in recording operation, it is located father away from the range immediately after the erasing operation. Accordingly, the conventional arrangement has drawbacks that it is difficult to read synchronous timing information and address information, both obtained by reproducing the pre-formatted sections 3703 and 3707 immediately after each of the recording and erasing operations; therefore each predetermined synchronous timing can not be detected; and further, it can not be identified whether the address is a predetermined one or not. Consequently, it is impossible to properly perform each of recording, reproducing and erasing operations, and this disadvantage is particularly remarkable immediately after the erasing operation.
The above problem may be solved by taking into consideration transient response time of the high-pass filter and by disposing the pre-formatted sections 3703, 3707 with intervals corresponding to a period of time sufficiently longer than the transient response time. However, the above arrangement causes lowering of transfer speed for information since the intervals between the pre-formatted sections become greater, and also fails to provide high density in recording information, and thus it can not give a decisive solution to the problem. Moreover, by minimizing the time constant of the high-pass filter, the transient response time can be shortened, thereby giving a solution to the above problem. However, such an arrangement causes phase shifts of data in the reproduced signals, thereby resulting in errors in reproducing operation. Accordingly, the time constant of the high-pass filters has its lower limit, and it is difficult to make the time constant lower than the limit.
Referring to FIG. 87, the following description will discuss how the AGC amplifier influence the output signal 3801 in the case where information recorded in the pre-formatted sections can not be read immediately after each of recording and erasing operations.
The AGC amplifier automatically adjusts the amplification degree of the reproduced signal S1 (S2), and responds to the recording or erasing level. Accordingly, when a signal shown in FIG. 88(b) is entered to the AGC amplifier, the amplification degree of the signal gradually decreases to a very low level in comparison with a proper amplification degree for normal reproducing operation. Following the process, as shown in FIG. 88(c), the level of the output signal of the AGC amplifier gets lowered extremely. As mentioned above, since the amplification degree of the AGC amplifier is not restored to an original one instantaneously, the amplitude of the reproduced signal upon reproducing the pre-formatted sections 3703, 3707 becomes very small immediately after the output signal 3801 of the AGC amplifier reached a steady state, and therefore reading operation for information can not be performed properly.
Accordingly, as with the case described referring to FIG. 87, also in this case, the conventional arrangement has drawbacks that it is difficult to read synchronous timing information and address information, both obtained by reproducing the pre-formatted sections 3703 and 3707 immediately after each of the recording and erasing operations; therefore each predetermined synchronous timing can not be detected; and further, it can not be identified whether the address is a predetermined one or not. Consequently, it is impossible to properly perform each of recording, reproducing and erasing operations. Additionally, the above problem may be solved by taking into consideration response time of the AGC amplifier and by disposing the pre-formatted sections 3703, 3707 with intervals corresponding to a period of time sufficiently longer than the response time. However, the above arrangement causes lowering of transfer speed for information since the intervals between the pre-formatted sections become greater, and also fails to provide high density in recording information, and thus it can not give a decisive solution to the problem.
As described referring to FIG. 87 and FIG. 88, the conventional device can not properly provide each of recording, reproducing and erasing operations for information on and from a desired sector, and fails to provide high transfer speed and high density for information as a magneto-optical recording and reproducing device.